[PATCH v3 00/10] merge-ort: implement support for packing objects together

This series implements support for a new merge-tree option, `--write-pack`, which causes any newly-written objects to be packed together instead of being stored individually as loose.

The notable change from last time is in response to a suggestion[1] from Junio to factor out an abstract bulk-checkin "source", which ended up reducing the duplication between a couple of functions in the earlier round by a significant degree.

Beyond that, the changes since last time can be viewed in the range-diff below. Thanks in advance for any review!

[1]: https://lore.kernel.org/git/xmqq5y34wu5f.fsf@gitster.g/

Taylor Blau (10):
  bulk-checkin: factor out `format_object_header_hash()`
  bulk-checkin: factor out `prepare_checkpoint()`
  bulk-checkin: factor out `truncate_checkpoint()`
  bulk-checkin: factor out `finalize_checkpoint()`
  bulk-checkin: extract abstract `bulk_checkin_source`
  bulk-checkin: implement `SOURCE_INCORE` mode for `bulk_checkin_source`
  bulk-checkin: generify `stream_blob_to_pack()` for arbitrary types
  bulk-checkin: introduce `index_blob_bulk_checkin_incore()`
  bulk-checkin: introduce `index_tree_bulk_checkin_incore()`
  builtin/merge-tree.c: implement support for `--write-pack`

 Documentation/git-merge-tree.txt |   4 +
 builtin/merge-tree.c             |   5 +
 bulk-checkin.c                   | 288 +++++++++++++++++++++++++------
 bulk-checkin.h                   |   8 +
 merge-ort.c                      |  42 ++++-
 merge-recursive.h                |   1 +
 t/t4301-merge-tree-write-tree.sh |  93 ++++++++++
 7 files changed, 381 insertions(+), 60 deletions(-)

Range-diff against v2:
 1:  edf1cbafc1 =  1:  2dffa45183 bulk-checkin: factor out `format_object_header_hash()`
 2:  b3f89d5853 =  2:  7a10dc794a bulk-checkin: factor out `prepare_checkpoint()`
 3:  abe4fb0a59 =  3:  20c32d2178 bulk-checkin: factor out `truncate_checkpoint()`
 4:  0b855a6eb7 !  4:  893051d0b7 bulk-checkin: factor our `finalize_checkpoint()`
    @@ Metadata
     Author: Taylor Blau <me@ttaylorr.com>
     
      ## Commit message ##
    -    bulk-checkin: factor our `finalize_checkpoint()`
    +    bulk-checkin: factor out `finalize_checkpoint()`
     
         In a similar spirit as previous commits, factor out the routine to
         finalize the just-written object from the bulk-checkin mechanism.
 -:  ---------- >  5:  da52ec8380 bulk-checkin: extract abstract `bulk_checkin_source`
 -:  ---------- >  6:  4e9bac5bc1 bulk-checkin: implement `SOURCE_INCORE` mode for `bulk_checkin_source`
 -:  ---------- >  7:  04ec74e357 bulk-checkin: generify `stream_blob_to_pack()` for arbitrary types
 5:  239bf39bfb !  8:  8667b76365 bulk-checkin: introduce `index_blob_bulk_checkin_incore()`
    @@ Commit message
         entrypoint delegates to `deflate_blob_to_pack_incore()`, which is
         responsible for formatting the pack header and then deflating the
         contents into the pack. The latter is accomplished by calling
    -    deflate_blob_contents_to_pack_incore(), which takes advantage of the
    -    earlier refactoring and is responsible for writing the object to the
    +    deflate_obj_contents_to_pack_incore(), which takes advantage of the
    +    earlier refactorings and is responsible for writing the object to the
         pack and handling any overage from pack.packSizeLimit.
     
         The bulk of the new functionality is implemented in the function
    -    `stream_obj_to_pack_incore()`, which is a generic implementation for
    -    writing objects of arbitrary type (whose contents we can fit in-core)
    -    into a bulk-checkin pack.
    -
    -    The new function shares an unfortunate degree of similarity to the
    -    existing `stream_blob_to_pack()` function. But DRY-ing up these two
    -    would likely be more trouble than it's worth, since the latter has to
    -    deal with reading and writing the contents of the object.
    +    `stream_obj_to_pack()`, which can handle streaming objects from memory
    +    to the bulk-checkin pack as a result of the earlier refactoring.
     
         Consistent with the rest of the bulk-checkin mechanism, there are no
         direct tests here. In future commits when we expose this new
    @@ Commit message
         Signed-off-by: Taylor Blau <me@ttaylorr.com>
     
      ## bulk-checkin.c ##
    -@@ bulk-checkin.c: static int already_written(struct bulk_checkin_packfile *state, struct object_id
    - 	return 0;
    - }
    - 
    -+static int stream_obj_to_pack_incore(struct bulk_checkin_packfile *state,
    -+				     git_hash_ctx *ctx,
    -+				     off_t *already_hashed_to,
    -+				     const void *buf, size_t size,
    -+				     enum object_type type,
    -+				     const char *path, unsigned flags)
    -+{
    -+	git_zstream s;
    -+	unsigned char obuf[16384];
    -+	unsigned hdrlen;
    -+	int status = Z_OK;
    -+	int write_object = (flags & HASH_WRITE_OBJECT);
    -+
    -+	git_deflate_init(&s, pack_compression_level);
    -+
    -+	hdrlen = encode_in_pack_object_header(obuf, sizeof(obuf), type, size);
    -+	s.next_out = obuf + hdrlen;
    -+	s.avail_out = sizeof(obuf) - hdrlen;
    -+
    -+	if (*already_hashed_to < size) {
    -+		size_t hsize = size - *already_hashed_to;
    -+		if (hsize) {
    -+			the_hash_algo->update_fn(ctx, buf, hsize);
    -+		}
    -+		*already_hashed_to = size;
    -+	}
    -+	s.next_in = (void *)buf;
    -+	s.avail_in = size;
    -+
    -+	while (status != Z_STREAM_END) {
    -+		status = git_deflate(&s, Z_FINISH);
    -+		if (!s.avail_out || status == Z_STREAM_END) {
    -+			if (write_object) {
    -+				size_t written = s.next_out - obuf;
    -+
    -+				/* would we bust the size limit? */
    -+				if (state->nr_written &&
    -+				    pack_size_limit_cfg &&
    -+				    pack_size_limit_cfg < state->offset + written) {
    -+					git_deflate_abort(&s);
    -+					return -1;
    -+				}
    -+
    -+				hashwrite(state->f, obuf, written);
    -+				state->offset += written;
    -+			}
    -+			s.next_out = obuf;
    -+			s.avail_out = sizeof(obuf);
    -+		}
    -+
    -+		switch (status) {
    -+		case Z_OK:
    -+		case Z_BUF_ERROR:
    -+		case Z_STREAM_END:
    -+			continue;
    -+		default:
    -+			die("unexpected deflate failure: %d", status);
    -+		}
    -+	}
    -+	git_deflate_end(&s);
    -+	return 0;
    -+}
    -+
    - /*
    -  * Read the contents from fd for size bytes, streaming it to the
    -  * packfile in state while updating the hash in ctx. Signal a failure
     @@ bulk-checkin.c: static void finalize_checkpoint(struct bulk_checkin_packfile *state,
      	}
      }
    @@ bulk-checkin.c: static void finalize_checkpoint(struct bulk_checkin_packfile *st
     +{
     +	struct pack_idx_entry *idx = NULL;
     +	off_t already_hashed_to = 0;
    ++	struct bulk_checkin_source source = {
    ++		.type = SOURCE_INCORE,
    ++		.buf = buf,
    ++		.size = size,
    ++		.read = 0,
    ++		.path = path,
    ++	};
     +
     +	/* Note: idx is non-NULL when we are writing */
     +	if (flags & HASH_WRITE_OBJECT)
    @@ bulk-checkin.c: static void finalize_checkpoint(struct bulk_checkin_packfile *st
     +
     +	while (1) {
     +		prepare_checkpoint(state, checkpoint, idx, flags);
    -+		if (!stream_obj_to_pack_incore(state, ctx, &already_hashed_to,
    -+					       buf, size, type, path, flags))
    ++
    ++		if (!stream_obj_to_pack(state, ctx, &already_hashed_to, &source,
    ++					type, flags))
     +			break;
     +		truncate_checkpoint(state, checkpoint, idx);
    ++		bulk_checkin_source_seek_to(&source, 0);
     +	}
     +
     +	finalize_checkpoint(state, ctx, checkpoint, idx, result_oid);
 6:  57613807d8 =  9:  cba043ef14 bulk-checkin: introduce `index_tree_bulk_checkin_incore()`
 7:  f21400f56c = 10:  ae70508037 builtin/merge-tree.c: implement support for `--write-pack`